The present invention relates to the production of low particulate bisphenols and to the use of such bisphenols in the manufacturing of high-quality, low-particulate polycarbonates intended for optical-grade products.
There are two common methods for manufacturing polycarbonates, the interfacial method and the melt polycondensation method. The interfacial method involves the reaction of a dihydric aromatic compound with a carbonyl halide, such as between bisphenol-A and phosgene. A primary disadvantage of the interfacial method is the use of phosgene and the use of a large amount of solvent. The melt polycondensation method uses a transesterification reaction between a dihydric aromatic compound and a diester of carbonic acid, such as diphenyl carbonate. The melt method avoids the disadvantages of the interfacial method and also eliminates chlorine from the process which is desirable because chlorine can lead to a less consistent color in the polycarbonate.
Bisphenol-A is the preferred dihydric compound in the synthesis of polycarbonate, and as such, much attention has been directed at developing methods for the purification of bisphenol-A. U.S. Pat. No. 4,447,655 describes a method for the purification of bisphenol-A through the use of a water/bisphenol-A crystal slurry. U.S. Pat. No. 4,798,654 is directed at a distillation column whereby recycling of the distillate leads to purified bisphenol-A. U.S. Pat. No. 4,931,146 involves the purification of bisphenol-A with steam-stripping in a multi-tubular packed column. In general, processes for the production and purification of bisphenols are well known, and are described inter alia in U.S. Pat. Nos. 4,107,218; 4,294,994; 5,210,329; 5,243,093; 5,245,088; 5,288,926; 5,368,827; 5,786,522; and 5,874,644.
High quality polycarbonate that has low levels of particulate matter is desirable in the manufacturing of DVDxe2x80x3s, CD-ROMxe2x80x3s, ophthalmic lenses, or other optical-grade products. The micron-sized particles that can be introduced in the process of manufacturing polycarbonate have the undesirable effect of scattering light. In the case of optical disks, this scattering of light introduces noise. Therefore, methods of removing these particulates is extremely desirable and several patents are consequently directed at this objective.
U.S. Pat. No. 6,008,315 discloses a method for producing bisphenol-A that has low-particulate impurities by using a calcined metal filter. U.S. Pat. No. 6,197,917 discloses the use of a fluorine resin membrane to filter micron-sized particles from a molten mixture of bisphenol-A with a carbonic diester. Combining the bisphenol-A and carbonic diester increases the efficiency of filtering with the fluorine resin membrane.
A different approach to eliminating particulates in the manufacturing of polycarbonate via the melt polycondensation method is disclosed in U.S. Pat. No. 6,204,352. In this process, the entire apparatus that is used in the synthesis of the polycarbonate is made of various alloys of nickel or stainless steel. These alloys were developed to prevent the discoloration of polycarbonate and further refined to eliminate the presence of metallic particulates in the polycarbonate.
The present invention relates to a method of producing a low-particulate bisphenol-A stream that can be used in the synthesis of polycarbonate. The method comprises the following steps:(a) introducing into a desorber column containing a non-aggregate packing material an adduct of a dihydric aromatic compound and phenol;(b) providing an operating temperature range in the desorber column that is sufficiently high and an operating pressure in the column that is sufficiently low such that the adduct is distilled;(c) discharging from the desorber column a first stream containing substantially all of the phenol; and(d) discharging from the desorber column a second stream containing substantially all of the dihydric aromatic compound; whereby the second stream is substantially free of added particulate matter as compared to the adduct stream introduced to the column. The invention can be applied in the context of a vacuum distillation column, or may utilize a stripping gas which is introduced in countercurrent flow relative to the adduct. The resulting stream of dihydric aromatic compound can further be used in a method of producing optical-grade polycarbonate.